An oscillation frequency of a ring oscillator depends on changes of ambient temperatures and power supply voltages.
Due to this, techniques for achieving a constant oscillation frequency of the ring oscillator have been proposed. For example, a delay time of a delay element in the ring oscillator is controlled by controlling a power supply voltage supplied to a ring oscillator, which may result in constant oscillation frequency.
Japanese Patent Application Publication No. 2002-540668, for example, discloses a system which is provided with an oscillator, a delay circuit, and a loop controller. The system keeps a delay in a signal path in an integrated circuit (IC) constant. In the system, the oscillator generates a first reference signal. The delay circuit generates a second reference signal by delaying the first reference signal. The loop controller keeps a phase difference between the first reference signal and the second reference signal constant by adjusting electric power supplied to the IC.
The oscillator in the system includes a logic gate formed in the IC and a passive delay by which an output of the logic gate is fed back to an input. The delay circuit is adjusted to program the delay in the signal path through the IC.
However, the delay circuit that is programmably configured is not equivalent, in a precise sense, to a logic circuit because an output of the delay circuit varies. Moreover, according to a control method where a direct current signal is extracted from the phase difference by using a low-pass filter and where the direct current signal is converted into voltage by using a power amplifier, whereby the power supply voltage is fed back, the control accuracy of the power supply voltage has limitations because a fluctuation of the loop controller itself is not taken into consideration.
Japanese Patent Laid-open Publication No. 2006-187153 discloses a system which compares an oscillation signal output from a ring oscillator formed on a semiconductor integrated circuit with a triangular wave signal output from an oscillator provided in a DC-DC converter. The system changes the output voltage by changing a comparison voltage which is used for a comparison with an output voltage, based on a result of the comparison.
In the system, the semiconductor integrated circuit achieves a rate of operation corresponding to that of the triangular wave signal by matching the oscillation signal of the ring oscillator with the triangular wave signal which serves as a reference signal.
However, because a phase comparator and a loop filter are formed of analog circuits, the control accuracy of the power supply voltage has limitations. Moreover, a lack of versatility may be a problem because it is not easy to change operating conditions of the phase comparator and/or the loop filter when the phase comparator and/or the loop filter are used for a different controlled target with respect to a controlled target intended at a design stage.
Japanese Patent Laid-open Publication No. 2002-100967 discloses a system which is provided with a semiconductor integrated circuit, an input signal generation circuit, a monitor circuit, a delay detection circuit, and a power supply voltage control circuit. The input signal generation circuit changes a phase difference between a reference signal and an input signal in response to a control signal upon generation of the reference signal and the input signal from a clock signal. The monitor circuit has a power supply voltage versus delay characteristic which has an equivalent or similar value to a critical path of the semiconductor integrated circuit, and the monitor circuit propagates the input signal and outputs a delay signal that is delayed by a time equivalent or similar to the critical path. The delay detection circuit detects a delay in the delay signal relative to the reference signal. The power supply voltage control circuit controls power supply voltage supplied to the semiconductor integrated circuit and the monitor circuit based on a result of the detection.
Generally, a variation in the delay time of the delay element is not only caused by the variation in the power supply voltage but also caused by variations in the manufacturing processes, the environmental temperatures, or the like.
However, because the monitor circuit of a controlled target is disposed outside the semiconductor integrated circuit, the variation in the processes at manufacturing stages and the variation in the environmental temperatures when in use may influence the delay time of the delay element.
In line with a recent semiconductor manufacturing process, an accuracy demand for a delay time control of a delay element in a ring oscillator has been growing. Likewise, an accuracy demand for a power supply voltage control has been also growing.